Gandalf's essentially an angel, so I'm not sure concepts like dedicating one's life to something conventionally apply to him.

Well, a Maia, and while I think his life was dedicated to a particular cause, there are enough hints that it's not Gandalf himself who did the dedicating :-/ Though he certainly seemed to be perfectly fine with that.

antiheroes

I don't think so -- the hobbits are not "anti", they are unexpected heroes, but pretty straight heroes otherwise.